1. Field of the Invention
The present invention relates to a cylindrical sintered slug suitable for use as materials for plastic processing, by way of example, cold-extruding iron-based mechanical parts such as gears, and a method for making it.
2. Prior Art
When mechanical parts such as gears are manufactured by plastic deformation processing such as forging and extrusion, the materials or preforms used to this end are referred to as slugs. Most of mechanical parts such as gears are formed of a steel material and generally assume a cylindrical forms. Accordingly, the slugs for plastic deformation processing used to this end are often formed of a steel material in a cylindrical form.
In this connection, cylindrical slugs applied mainly to cold-compression deformation processings have been manufactured by the following techniques.
(1) A rod-like steel material is cut into a columnar shape, which is subsequently flattened, perforated and formed by cold plastic deformation processing. Afterwards, the formed product is subjected to annealing and plastic deformation processing with lubrication such as phosphating.
(2) A columnar member is cored out by hot forging and, subsequently, extruded partially machined or cut and formed. Afterwards, the formed product is annealed and lubricated.
(3) A columnar member is machined or cut.
However, such conventional techniques for making cylindrical slugs as mentioned above leave much to be desired because of their disadvantages of increased number of parts involved and the poor yield of material.
One may predict that sinter forging techniques relying upon powder metallurgy give cylindrical slugs with improved yields of material and great economical efficiency. However, as described in some literature, for instance, the "Sintered Mechanical Parts and their Design/Production", edited by the Japan Association of Powder Metallurgy and published by Gijutu Shoin, cylindrical slugs manufactured by sinter forging present the phenomenon that the amount of pores in their central regions are smaller than that of pores in their surface layers.
It is presumed that this phenomenon is caused by the fact that a larger number of pores remain on the surface of slug, because the result that the surface of slug preform in contact with the tool is cooled at the time of forging makes its plastic flowing difficult.
Accordingly, when such slugs are used for cold- or hot-compression plastic processings, a problem on as compressed arises. That is cracks or breaks are caused on their outer and inner surface because of the friction on their surface contact with each tool surface. For that reason, conventional sinter-forged slugs had to be machined or cut to remove their surface layers for plastic deformation processings use.
In this connection, it is noted that reducing the amount of pores in the sinter-forged slugs surface may be achieved by increasing forging temperature and pressure as well as tool temperature; however, the resulting slugs have a disadvantage of being reduced in their service life of productivity.
The situation being like this, the sinter-forging techniques have not yet been used to obtain slugs for making iron-based mechanical parts such as gears in spite of their improved economical efficiency.
The present invention, in view of the foregoing, seeks to provide a cylindrical, iron-based sintered slug for plastic processing, which has no crack or breaks on its surface and can be processed at lower costs but with higher yields of material.